Hi,
Besides charging up the battery, check your wiring and connections are all good and free of any corrosion. The current battery might be fine, just not getting charged.
Since it's the original positive ground setup, I'm assuming you have the orginal generator in the car. Those don't put out a lot of juice, so some folks like to use an Optima battery (or similar design from another manufaturer). That type of battery charges easily and holds up well.